In a closed loop fuel control system, the air-fuel ratio of the post combustion gases is sensed by an exhaust gas sensor to derive a feedback control signal which is used to control the ratio of air to fuel prior to combustion so that the air-fuel ratio of the exhaust gases is controlled within a narrow range of air-fuel ratios where the efficiency of three-way catalytic conversion is at a maximum.
In internal combustion engines of the type which completely cut off fuel supply during deceleration, the closed loop fuel control sense a lean condition which results in an enrichment signal during the fuel cut-off period. In response to the enrichment signal, the system supplies an excessively rich mixture at the instant when the fuel supply is restarted so that the air-fuel ratio within the exhaust system drifts from the desired range. It is therefore desirable to disable the closed loop fuel control during such fuel cut-off periods. However, a momentary throttling operation during transmission gear changes may be erroneously sensed as a deceleration to cut off the fuel. If the closed loop control is disabled during such operation, the system will oscillate abnormally.